While some experts believe that better sex education and access to pornography could explain this statistic, Dr. Helen Fisher, an anthropologist and Chief Scientific Advisor to Match.com, suggests millennials aren't sleeping with each other for two key reasons.

First, because millennials are more ambitious in their careers, and second, because they are more cautious about getting into relationships.

Today's young people are "a very ambitious generation," Fisher told Business Insider. She believes that millennials are focusing on their careers over sex and love, and holding off on having regular sex until they're ready to commit to a serious relationship.

"The more you have to offer a person in terms of education and earning power and social net worth, the more likely you are to find a partner of higher 'mate value,'" Fisher said. She describes 'mate value' as a person's worth in a relationship, measured by how much they can give to a partner romantically. Millennials could be avoiding sex because that's one of the first steps of a relationship, and they are looking "to increase their 'mate value' first," Fisher said.

Young people are also putting their careers ahead of getting married. "In the past, in order to be a grown-up, you really had to be married," the expert said. "This generation is not interested in getting married." While having an active sex life doesn't mean being interested in marriage, Fisher said that having sex makes people more susceptible to becoming attached and falling in love.

"When you have sex with someone you can catch the feelings of romantic love," she explained, adding that having an orgasm triggers hormones that inspire feelings of attachment, like oxytocin — a hormone released by both sexes when they orgasm. "The bottom line is, casual sex is not casual. It leads to relationships."

I asked Fisher about online dating apps and sites, and why it is young millennials are meeting more people online but having less sex. The biggest problem with online dating is "cognitive overload," she said. "The human brain can't cope with that many choices ... The more people you meet, the less likely you are to go out with any of them."

Millennials "are more cautious, more careful, more picky," and "more ambitious," she explained, adding that one reason young people are so selective is that many of them come from single-parent homes or have divorced parents, and want to avoid getting divorced themselves.

"The sex is going to come along, they know that. It's everywhere for them when they want it," Fisher said. "They're choosing not to have this because they're trying to do something else."